Response Of Photosynthesized Carbon Transfer Process To Grazing In Vegetation-Soil System Of Desert Steppe

The report provided by IPCC in 2019 encouraged to mitigate the continuously elevation in atmospheric CO₂ emissions by human activities.Grazing is the traditional way of utilization of grassland.And Inner Mongolia desert steppe as a fragile mid-temperate steppe ecosystem.Here,problems such as grassland degradation caused by grazing gradually conflict with the purpose of reducing CO₂ emissions.However,vegetation photosynthetic carbon is more sensitive than long-term carbon storage to external stimuli in the near future.In order to understand the effect of grazing on carbon transfer process in Inner Mongolia desert steppe.This study relies on the Mongolian horse grazing experimental platform established in 2014 in Xilamuren grassland.Three kinds of grazing intensity（enclosure,light grazing and heavy grazing）were set in the experiment.Combined with the methods of field sampling and laboratory analysis,¹³C in situ labeling technique was used to label the CO₂needed for community photosynthesis.Study on net productivity,ecosystem productivity and carbon storage of communities in plant-soil-microbial system under different grazing intensity.Determination of ¹³C content and recovery rate of ¹³C in plants,roots,soil and soil-microorganisms in the system,as well as ¹³C content of soil respiration.In addition,taking advantage of the fractionation differences produced by different photosynthetic pathways in the soil,theδ¹³C microbial biomass method was used to separate soil respiration.The contribution of carbon release from four different sources（pure root respiration,rhizosphere microbial respiration,stimulating carbon release,soil organic matter decomposition）to total soil respiration was analyzed.And understand the response of carbon release from the above four sources to grazing intensity.Furthermore,the environmental factors that may affect the above eigenvalues in the process of photosynthetic carbon transfer were analyzed.1.Carbon sequestration of plant communitiesThe growing season of desert steppe ecosystem in Inner Mongolia is carbon sink.Light grazing significantly increased the fixed CO₂ flux in desert steppe ecosystem（p<0.05）.With the increase of grazing intensity,the fixed CO₂ of desert steppe ecosystem decreased gradually.Due to the change of precipitation,the interannual effect of aboveground net primary productivity was significantly higher than that of grazing（p<0.001）.In drought years,light grazing will increase the aboveground net primary productivity of vegetation（light grazing>heavy grazing>enclosure）,while grazing in rainy years will not increase the aboveground net primary productivity of vegetation.When using the method of ecosystem respirator box,the interaction between grazing and soil environmental factors significantly affected the total ecosystem productivity（p<0.01）,ecosystem respiration（p<0.01）and net CO₂ exchange capacity（p<0.05）.Light grazing could increase the carbon storage of 0～20 cm soil organic carbon pool,but decreased with the increase of grazing intensity（light grazing>enclosed area>heavy grazing）.2.Carbon distribution of plant communitiesDuring the 35-day follow-up period,the average distribution ratio of photosynthetic carbon in plant-soil system was 6:3:0.5.Among them,the aboveground plants were fixed with an average of 63%,62%and 57%;the root system fixed as 63%,62%and 57%respectively for 0～10 cm;the root system of 10～20 cm was fixed at 6.4%,5.3%and 5.4%,respectively;the recovery rates of ¹³C in the soil of 0～10 cm were 4.1%,3.9%and 2.5%,respectively;the soil of 10～20 cm was fixed at 1.1%,0.8%and 0.9%,respectively.With the increase of grazing intensity,the root ¹³C recovery rate and recovery speed of plant community increased.In addition,light grazing significantly increased the content of¹³C in microbial biomass carbon in 0～10 cm soil（p<0.05）.The distribution rate of photosynthetic carbon in root carbon pool and soil carbon pool is very fast.Basically,the enriched micro-biomass ¹³C,which was significantly different with the both side of the ex-root net,during the whole tracking period（p<0.05）.This is because photosynthetic carbon is exchanged in soil mainly in the form of root exudates.Mongolian horses and sheep have different dietary preferences.Compared with sheep,grazing Mongolian horses changed the carbon allocation strategy of plant communities:a larger proportion of carbon was fixed in the carbon pool tissue（root）,rather than carbon source tissue（leaves,stems）.Moreover,with the increase of grazing intensity,the plant put more new photosynthetic carbon into the root system.The pressure difference of air saturated water vapor can reflect the carbon distribution strategy of the plant community.With the increase of grazing intensity,the air saturated water vapor pressure difference in the study area increases.The greater VPD,the faster the carbon allocated to the roots by the vegetation.3.Carbon release from plant communitiesLight grazing in summer reduced the total Soil respiration flux of desert steppe ecosystem in Inner Mongolia.However,enclosure increased the flux（enclosure>heavy grazing>light grazing）（p>0.05）.The increase in soil respiration flux caused by grazing is due to the contribution of increased underground net primary productivity to soil respiration.Light grazing increased pure root respiration,while heavy grazing increased CO₂ released by soil organic matter decomposition.The contribution of four carbon sources to soil total respiration respectively was pure root respiration,soil organic matter decomposition,rhizosphere microbial respiration and priming decomposition of soil organic matter:Enclosure（62%,38%,0.86%,0.72%）;light grazing（67%,32%,0.56%,0.43%）;heavy grazing（53%,47%,0.72%,0.52%）.Through redundancy analysis,it was found that soil surface temperature was the main environmental factor to explain the four different soil respiratory fluxes（p<0.01,R²=0.40）.The increase of root litter caused a negative priming effect in the desert steppe ecosystem of Inner Mongolia,it means that the input of exogenous carbon accelerated the sequestration of soil organic matter in the steppe.There was a significant negative correlation between rhizosphere microbial respiration and priming decomposition of soil organic matter（p<0.001,R²=0.99）.With the increase of root respiration（pure root respiration and rhizosphere microbial respiration）,the CO₂ produced by soil organic matter decomposition decreased.When the input of root carbon storage increased,the CO₂produced by soil organic matter decomposition will also decrease.In summary,grazing Mongolian horses in the desert steppe of Inner Mongolia will change the allocation strategy of photosynthetic carbon in the plant-soil-microbial system.The ratio allocated to plant-roots-soil averages about 6:3:0.5.Plant communities have different coping strategies for different intensities and livestock grazing,which in turn affect the sequestration of soil organic carbon pools.Affected by microbial communities and soil temperature,carbon release from various sources in soil will respond to different grazing intensities.